1 Answer
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Note that the distribution of the random variable $Y$ which is $0$ with probability $0.2$ and $1$ with probability $0.8$ has the same mgf: to check, compute. Now use uniqueness.

Like in the case of the Laplace Transform, we often recognize an mgf as being a familiar one, and thereby identify a distribution.

Detail: Let $Y$ be a Bernoulli random variable which is $0$ with probability $a$, and $1$ with probability $b$. Then
$$E(e^{tY})=a e^{(0)t}+be^{(1)(t)}=a+be^t.$$
Conversely, by the uniqueness theorem, a random variable whose distribution has mgf $\,a+be^t$ is a Bernoulli random variable, taking on the value $0$ with probability $a$, and $1$ with probability $b$.

We have found that our random variable $X$ has distribution with mgf $\,0.2+0.8e^t$. Therefore $X$ must be a Bernoulli random variable, with $\Pr(X=0)=0.2$ and $\Pr(X=1)=0.8$.

Andre, thanks for your response, but I don't feel it answered my question. I'm not seeing how the answers were obtained. Assuming you didn't know that $P(X=0) = 0.2$ and $P(X=1) = 0.8$, how would you go about solving for those two probabilities?
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konfushusSep 21 '12 at 3:30

You would recognize that $a+be^t$ is the mgf of a distribution which takes on value $0$ with probability $a$ and $1$ with probability $b$. Then you read off $a$ and $b$ from the mgf you computed.
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André NicolasSep 21 '12 at 3:33

Sorry Andre, I'm still not following. I want to see the answer mathematically, not through just "looking at it". If looking at $P(X=0)$ first, let me plug that into the equation above. That leaves me with, $$e^{t(0)}p_X(0) = 0.2 + 0.8e^t$$ In order for $p_X(0) = 0.2$, the $0.8e^t$ must disappear. That's what I'm not seeing. Where does it go? Thanks again for your help.
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konfushusSep 22 '12 at 22:35

@konfushus: I have added to the post, but really only repeating in symbols what was written before.
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André NicolasSep 22 '12 at 23:25

Andre, thanks for the added detail. I see now that the question wanted me to recognize that the random variable in question is, in fact, a Bernoulli random variable, thus having a sample space of just 0 and 1. If plugging 0 and 1 into the LHS and comparing like co-efficients, I see how the solution is obtained. Thanks again for your help.
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konfushusSep 23 '12 at 14:34